This proposal is to study the biosynthesis, function, regulation, and possible role in bacterial pathogenesis of two modifications that alter the membrane phospholipid bilayers of bacteria. Both modifications alter the cis double bonds of the unsaturated acyl chains of the lipids after the lipids have been synthesized, deposited, and are functioning in the cell membranes. One of these modifications, cis-trans isomerization, is limited to a few families of bacteria whereas the other, cyclopropane ring formation is found in a very wide range of diverse bacteria. Both lipid modifications increase the resistance of the bacterium to environmental stresses. For example, cyclopropane ring formation increases resistance to acid conditions and may be of importance in survival of a water-borne pathogen as it passes through the stomach. Cis-trans isomerization results in improved fitness for growth at high temperature and resistance to certain toxic compounds. Both modifications are conserved in numerous pathogens and cyclopropane ring formation has been ascribed a role in Mycobacterium tuberculosis pathogenesis. These processes will be studied by a combination of biochemical, genetic, and molecular biological approaches including tests of pathogenesis in a mouse model system